Gas chromatography involves physically separating constituents of a sample in a carrier gas, which flows through a column, and measuring the separated constituents. A pulse of the sample is injected into the flow of the carrier gas and the constituents interact with a stationary phase material in a column. At the end of the column the individual components are more or less separated in time. Detection of the carrier with the separated constituents provides a pattern of retention times, which, by calibration or comparison with known samples, indicates the constituents of the sample qualitatively and quantitatively. The main components of such a system are the column, an injector with a mixing chamber for introducing the sample into the carrier, a detector at the outlet end of the column, fluid controls, and a computer for treating and displaying the output of the detector. The display generally shows the height of each peak verses its retention time. An oven generally is used to elevate temperature to maintain the sample in a volatile state, and to improve the discrimination of constituents. A typical gas chromatographic system is disclosed in U.S. Pat. No. 5,476,000
It is often desirable to be able to predict how retention times will change in response to changes in the column temperature and inlet carrier gas pressure. Several cases of this are listed below:
For example, a user of one gas chromatography system may want to utilize a method developed on another system and get retention times the same, or nearly the same, as those obtained on the other system for the same sample constituents. Even when the two gas chromatographs are very accurately calibrated, this has generally not been possible due to differences in column geometry from one column to another of the same type.
Alternatively, a user may want to simulate effects of changes in temperature and pressure in order to optimize their values for achieving satisfactory separation between the retention times of the various possible sample constituents in the minimum analysis time. A means of achieving this is also described in the accompanying patent application. In addition to this invention, there are products on the market which also claim to be able to do this.
Yet another reason for wanting to be able to make such predictions is to transfer a method from one column to another of a different geometry, possibly using a different carrier gas, and get the same retention time pattern. Means for achieving this are described, for example, in U.S. Pat. No. 5,405,432.
Other situations in which it is desirable to be able to predict the effect of changes in pressure on retention times involve changes in column outlet pressure, which is most commonly atmospheric pressure. One of these situations involves naturally occurring fluctuations in atmospheric pressure. Compensation for outlet pressure makes retention times more constant from one run to the next on a given system. U.S. Pat. No. 5,476,000 describes a means for doing this. Another situation is the use of the same method on two different GC systems run at significantly different elevations. Compensation for the differences in atmospheric pressure must be made in order to get the same retention times on the two systems.
Another type of situation involving outlet pressure is the transfer of a method developed with one detector to systems using another detector operated at a different pressure. For example, a method may be developed using a mass spectrometer detector involving a near zero outlet pressure, and then used routinely on systems using detectors with atmospheric outlet pressure. In each of the above cases, it is desirable to be able to accurately calculate the change in operating conditions that will compensate for the change or difference in outlet pressure.
The carrier gas holdup time is the time it takes a small segment of carrier gas to flow from one end of the column to the other. At constant temperature, the constituent retention times are proportional to the carrier gas holdup time. With temperature programming, there is a similar but more complex dependence on the holdup time. Because of this, all of the examples and situations listed above require the ability to accurately predict changes in carrier gas holdup times accompanying changes in column temperature and inlet pressure.